1. Field of the Invention
The present invention relates to an optically readable identification mark member and a card using such an optical identification mark member, and more particularly to a card such as a credit card, a prepaid card in which a seal for prevention of fake product or fake prevention treatment is provided.
2. Description of the Related Art
There has been disclosed a technique in which diffraction grating patterns are added to a card as information recording means or these identification marks are optically read, thereby the card is identified. Published Unexamined Japanese Patent Application (PUJPA) 3-71383 (Nishiruma et al.) discloses an optical identification mark using a holographic diffraction grating, and its reading technique.
FIG. 1 shows one example of diffraction grating patterns based on the concept disclosed in the above PUJPA 3-71383. The identification mark comprises a plurality of mark elements such as four mark elements 1a to 1d as shown in the drawing. Each mark element has its own diffraction grating pattern. Such a diffraction grating pattern can be produced by, for example, using a two-beam laser and exposing a photosensitive film with fine interference fringes generated by interference of the two beams.
Reading the mark elements is performed by radiating the laser beam to each element sequentially from the right above and detecting the diffracted light by a plurality of sensors, such as photodiodes. Then, one signal is formed by the diffracted light from each element, and the identification mark, which is formed of a set of four element, is discriminated by the types of plurality of signals, for example, four signals as shown in the drawing, and the order thereof. The own signal, which each element has, is determined based on the principle to be explained as follows.
A first factor concerns a relationship between the direction of the grating pattern and that of the diffracted light. The diffracted light returns to a direction perpendicular to the lines of the gratings. Due to this, if the directions of the grating of elements are different from each other at 90.degree., as in the case between the elements 1a and 1b in FIG. 1, the directions to which the diffracted light return are different from each other at 90.degree. in the plane. With reference to FIGS. 6 and 7, the relationship between the direction of the grating and that of the diffracted light can be understood, though they show the outline of an apparatus used in the present invention.
A second factor concerns a relationship between a spatial frequency of the pattern grating, that is, pitches of lines, and the diffraction angle of the diffracted light. As the spatial frequency of the grating is higher, that is, the pitches of the lines are narrower, the diffraction angle becomes larger. Therefore, as in the case between the elements 1b and 1c in FIG. 1, if the pitches of the lines of the gratings of elements are different from each other, the diffraction angles of the respective diffracted light differ from each other. With reference to FIGS. 16 and 17, the relationship between the spatial frequency of the grating and the diffraction angle of the diffracted light can be understood, though they show the outline of an other apparatus used in the present invention.
However, as shown in FIG. 1, in a case where an identification mark is formed of mark elements having diffraction grating patterns, following problems occur.
Specifically, if the types of patterns are increased in order to increase the types of signals which the mark elements can obtain, the number of sensors (e.g., photodiodes) must be increased in proportion to the number of patterns. If the number of sensors is increased, there occur problems in that the cost of the reading apparatus is increased and reading errors are easily generated.